Development of a semi-active rotary damper has been considerably advanced. A semi-active rotary damper embodied using a working fluid such as an electrorheological or magnetorheological fluid whose properties are changed by changing intensity of an electric or magnetic field does not require complicated mechanical elements.
When the electrorheological or magnetorheological fluid is subject to an electric or magnetic field, particles contained in the fluid are connected to form chains resulting in that the fluid is changed from a liquid phase to a gel phase. In non-electric/magnetic field, the fluid acts as a Newtonian fluid whose shearing stress is proportional to strain rate. In an electric or magnetic field, however, the fluid acts as a Bingham fluid whose initial stress without strain is equal to a yielding stress because particles scattered in the fluid is rearranged to form chains.
A conventional rotary damper using a magnetorheological fluid obtains a damping torque in a directive shearing mode. As showed FIG. 6, the rotary damper 10 comprises a circular plated rotor 12 fixed to a damping shaft 11, and a couple of annular stators 13 and 14 disposed at two opposed sides of the rotor 12. The rotor 12 has a rim fixed at its periphery to have "T" shaped section. The stators 13 and 14 are connected with an annular non-magnetoconductive member interposed so that a space 16 for receiving the rotor 12 is formed. The space 16 is sealed by sealing members 17 and 18, and filled with the magnetorheological fluid. The stators 13 and 14 act as electromagnetic poles when the electric current flows through coil 21 and 22. The stators 13 and 14 have opposed polarities to form a magnetic field in the space 16.
Before forming the magnetic field, the magnetorheological fluid has a state as showed in FIG. 7A, while, after forming the magnetic field, the fluid has a state as showed in FIG. 7B. In FIG. 7A and FIG. 7B, reference numbers 23 and 24 are designated to the magnetic poles, and 25 is designated to magnetotactic particles contained in the magnetorheological fluid.
When a torque is transmitted to the damping shaft 11 of the rotary damper from a foreign rotating shaft, rotor 12 also rotates. To break rotating motion of the rotating shaft, the electric current is switched on through the coils 21 and 22 so that the magnetorheological fluid changed to a state as showed in FIG. 7B, whereby viscosity of the fluid is heightened. Thus, a frictional force between the circular plated rotor 12 and the magnetorheological fluid is enlarged to break rotation of the rotating shaft.
However, breaking force obtained in the directive shearing mode is not so large.